1. Field of the Invention
The present invention relates to a scanning electron microscope which scans an electron beam on a surface of a specimen, detects secondary electrons generated from the specimen, and thereby obtains a scanning image of the specimen surface.
2. Description of the Related Art
In recent years, as semiconductor devices have been becoming more highly integrated and finer, scanning electron microscopes have started to be used, in place of optical microscopes, in dimension measurement of and shape evaluation of semiconductor devices in manufacturing processes of the semiconductor devices. In order to automatically and speedily process measurement of mass-manufactured semiconductors, it is necessary to speedily detect each measurement point on a wafer, and, for that purpose, it is necessary to focus an electron beam on a pattern speedily after the electron beam has moved to the each measurement point.
In an electron optical system, a condition for focusing on a wafer is uniquely determined by: an accelerating voltage of electrons irradiated to the wafer; and a height of the wafer. Additionally, the accelerating voltage of the electrons is determined by: an extracting voltage of the electrons; a retarding voltage applied to the wafer so as to decelerate the electrons; and an electrostatic voltage of a surface of the wafer. In order to obtain a desired accelerating voltage, in normal cases, the retarding voltage is controlled in accordance with the electrostatic voltage of the wafer on condition that the extracting voltage has been maintained at a constant level. For example, by measuring a potential of the wafer by use of an electrostatic potential meter grounded outside a specimen chamber, and canceling an influence from the electrostatic voltage of the wafer by feeding back a result of the measurement to the control over the retarding voltage, the accelerating voltage can be maintained at a constant level and thereby can be prevented from affecting the condition for the focusing. On the other hand, in order to measure the wafer height, there has been adopted a method as described in, for example, Japanese Patent Application Laid-open No. Hei11-126573, the method including the steps of: irradiating laser light to the wafer; detecting the wafer height by utilizing reflected light thereof; feeding back, to an objective lens which is one of the electron optical systems, height information obtained thereby; and, at the same time as movement of the electron beam to a measurement point is completed, applying excitation that is necessary for the focusing to the objective lens. By performing the above control over the accelerating voltage and the wafer height at one time, it becomes possible to focus an electron beam at the same time as movement of the electron beam to any measurement point is completed.
However, in recent years, there have been occasionally found wafers each showing a change in electrostatic voltage of a surface thereof when being moved between the outside and the inside of a specimen chamber. For example, there is a wafer that assumes hardly any voltage outside a specimen chamber, but assumes a voltage of several ten to several hundred volts inside the specimen chamber. Such a difference in amount of electrostatic charge between the outside and the inside of the specimen chamber depends on a manufacturing environment intrinsic to the each wafer, and also on a thickness of layer electrically charged. Therefore, such differences are not necessarily constant among those wafers that have undergone the same manufacturing processes. As to each of such wafers, even if a focusing condition is determined on the basis of an electrostatic voltage of the wafer measured by use of an electrostatic potential meter grounded outside a specimen chamber, the focusing condition changes when the electrostatic voltage becomes different inside the specimen chamber. Consequently, because an electron beam cannot be focused in a measurement point, detection of the measurement point ends in failure. Eventually, a supportive operation by an operator comes to be required as the measurement cannot be automatically performed.
As means for solving such a problem, there is, for example, a retarding focus system. As has been mentioned above, a condition for focusing on a wafer is determined by an accelerating voltage of electrons irradiated to the wafer, and a height of the wafer, and this means that, on condition that the wafer height is accurately measured, an accelerating voltage at the time when the wafer is focused is uniquely determined. Accordingly, by changing the focusing condition by changing a retarding voltage thereto with an extracting voltage of electrons being maintained at a constant level, a potential on a surface of the wafer at a measurement point can be back calculated from values of the accelerating voltage, the extracting voltage and the retarding voltage at the time when the wafer is focused. Additionally, there has been a method as described in Japanese Patent Application Laid-open No. 2001-52642 including the steps of: installing plural electrostatic potential meters in a place approximate to a specimen inside a specimen chamber; and feeding back values obtained based on results of measurement thereby to the retarding voltage.